High Near-field Enhancement Research Articles

Au Nanorings/TiO2 NPs@MXene-Based Metasurfaces with a Magnetic Mirror-Modulated ECL Strategy for Extracellular Vesicle Detection.

A metasurface as an artificial electromagnetic structure can concentrate optical energy into nanometric volumes to strongly enhance the light-matter interaction, which has been becoming a powerful platform for optical sensing, nonlinear effects, and quantum optics. Herein, we developed a novel hybrid plasmonic-dielectric metasurface consisting of Au nanorings (Au NRs) and TiO2 nanoparticles derived from MXene (TiO2 NPs@MXene). The hybrid metasurface simultaneously benefited from the high near-field enhancement effect of plasmonic materials and the low loss of dielectric materials. Furthermore, the optical modulation efficiency of the hybrid metasurface can be regulated by a magnetic mirror configuration. The magnetic mirror acted like a mirror, confining the electrons to a limited region and increasing the density of the surface plasmon. Moreover, the electrochemiluminescence (ECL) of the Cu2BDC metal-organic framework (Cu2BDC-MOF) served as a light source for the Au NRs/TiO2 NPs@MXene metasurface. Due to the exceptional light manipulation capability of the hybrid metasurface and the coordination of the magnetic mirror, the isotropic ECL signal can be dynamically amplified and converted into polarized emission. Finally, a metasurface-regulated ECL (MECL)-based biosensor with a dual-positive membrane protein recognition strategy was developed for the accurate identification of gastric cancer-derived extracellular vesicles. The novel MECL research opened up a new route in the realization of dynamically tunable metasurfaces for optical sensing and novel nanophotonic devices.

Passive trapping of biomolecules in hotspots with all-dielectric terahertz metamaterials

Electromagnetic metamaterials feature the capability of squeezing photons into hotspot regions of high intensity near-field enhancement for strong light-matter interaction, underpinning the next generation of emerging biosensors. However, randomly dispersed biomolecules around the hotspots lead to weak interactions. Here, we demonstrate an all-silicon dielectric terahertz metamaterial sensor design capable of passively trapping biomoleculars into the resonant cavities confined with powerful electric field. Specifically, multiple controllable high-quality factor resonances driven by bound states in the continuum (BIC) are realized by employing longitudinal symmetry breaking. The dielectric metamaterial sensor with nearly 15.2 experimental figure-of-merit enabling qualitative and quantitative identification of different amino acids by delivering biomolecules to the hotspots for strong light-matter interactions. It is envisioned that the presented strategy will enlighten high-performance meta-sensors design from microwaves to visible frequencies, and serve as a potential platform for microfluidic sensing, biomolecular capture, and sorting devices.

Optical anapole modes in hybrid metal–dielectric nanoantenna for near-field enhancement and optical sensing

Metal–dielectric nanostructures in the optical anapole modes are essential for light–matter interactions due to the low material loss and high near-field enhancement. Herein, a hybrid metal–dielectric nanoantenna composed of six wedge-shaped gold (Au) nanoblocks as well as silica (SiO2) and silicon (Si) nanodiscs is designed and analyzed by the finite element method (FEM). The nanoantenna exhibits flexibility in excitation and manipulation of the anapole mode through the strong coupling between the metal and dielectrics, consequently improving the near-field enhancement at the gap. By systematically optimizing the structural parameters, the electric field enhancement factors at wavelengths corresponding to the anapole modes (AM1 and AM2) can be increased to 518 and 1482, respectively. Moreover, the nanoantenna delivers great performance in optical sensing such as a sensitivity of 550 nm/RIU. The results provide guidance and insights into enhancing the coupling between metals and dielectrics for applications such as surface-enhanced Raman scattering and optical sensing.

Efficient Excitation and Tuning of Multi-Fano Resonances with High Q-Factor in All-Dielectric Metasurfaces.

Exciting Fano resonance can improve the quality factor (Q-factor) and enhance the light energy utilization rate of optical devices. However, due to the large inherent loss of metals and the limitation of phase matching, traditional optical devices based on surface plasmon resonance cannot obtain a larger Q-factor. In this study, a silicon square-hole nano disk (SHND) array device is proposed and studied numerically. The results show that, by breaking the symmetry of the SHND structure and transforming an ideal bound state in the continuum (BIC) with an infinite Q-factor into a quasi-BIC with a finite Q-factor, three Fano resonances can be realized. The calculation results also show that the three Fano resonances with narrow linewidth can produce significant local electric and magnetic field enhancements: the highest Q-factor value reaches 35,837, and the modulation depth of those Fano resonances can reach almost 100%. Considering these properties, the SHND structure realizes multi-Fano resonances with a high Q-factor, narrow line width, large modulation depth and high near-field enhancement, which could provide a new method for applications such as multi-wavelength communications, lasing, and nonlinear optical devices.

High Near-Field Enhancement in Plasmonic Coupled Nanostructure for Spaser Application

We theoretically demonstrated a kind of plasmon coupled elliptical/crescent nanostructure to achieve a vast range of applications based on nanolaser or spaser with high intensity. To overcome the ohmic losses, the plasmon ellipse is composed of the gold film substrate with a gain media. A simple ellipse has been chosen from which a variety of dimer configurations have been formed by the symmetry alteration technique which is then tested for different light polarizations and gap variations. The proposed model supports localized surface plasmon resonance mode (LSPR). Moreover, the localized surface plasmon resonance (LSPR) property of the proposed nanostructure is numerically analysed by the finite-element method (FEM), and the results show that the electric field intensity (EFI) can be amplified to a large value by symmetry breaking in the elliptical nanostructure. Various plasmon modes can be excited by selecting the appropriate gain media. In addition to this, a compact tuneable multi-wavelength nanolaser (spaser) can be developed by using this model.

Theoretical Study of High Near-Field Enhancement Associated with the Lasing Action in Strongly Coupled Plasmonic Nanocavity Arrays

Lasing action in strongly coupled plasmonic nanocavity arrays was investigated both theoretically and experimentally. This phenomenon was generated by coupling a gain medium with a surface lattice ...

Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling

It has been shown in the past that fractal geometries are beneficial for radio and communication antenna designs in terms of bandwidth and gain. Recently, this concept was extended to plasmonic nanoantennas. Here, we present a fabrication method based on electron beam lithography and focused helium ion beam milling to further miniaturize dimer nanoantennas of 0th, 1st and 2nd order Sierpiński fractals. With this state-of-the-art approach, it becomes feasible to experimentally move their resonance conditions into the sub-micron wavelength regime, while maintaining excellent pattern definition and achieving sub-10 nm gap sizes for high near-field enhancement. These highly sophisticated nanostructures are numerically simulated and analyzed by dark-field scattering spectroscopy to monitor the effects of the fractal structuring on the scattering spectra and near-field enhancement.

Enhancement of extraordinary optical transmission and sensing performance through coupling between metal nanohole and nanoparticle arrays

The extraordinary optical transmission (EOT) of sub-wavelength nanohole array is significantly enhanced through introducing nanoparticles, including nanospheres and nanocylinders, into the centers of the nanoholes. Maxima of 56% and 48% for nanocylinder and nanosphere matrices are achieved, compared with that of 37% for a none-in-hole nanohole array gold film. The mechanism behind the phenomena is discussed, indicating that surface plasmon mode coupling between nanoholes and nanoparticles rather than Fabry–Pérot resonance is the cause for the EOT enhancement. High near-field intensity enhancement also leads to the interaction between analytes and optical field increasing, therefore an improved sensitivity and figure of merit for biosensing. The modified structures are highly promising in practical sensing applications due to the incident angle independence. Similar results are obtained for a hexagonal array of nanohole gold film and nanoparticle modifications thereof.

Simulations of the Near-Field Enhancement on AFM Tip Irradiated by Annular Laser Beam

The near-field enhancement underneath the AFM tip irradiated by a laser beam is widely utilized in various nanofabrication techniques, such as nano-welding, nano-manipulation, nano-etching and so on. In this paper, a numerical model of near-field enhancement on AFM tip irradiated by annular laser beam is established using COMSOL Multiphysics. The results show that when the AFM tip is irradiated by annular laser beam, strong near-field enhancement is induced at the apex of the tip. The field distribution curve with the substrate presents a saddle shape, which demonstrates that the annular beam irradiated the AFM tip produce an extremely high near-field enhancement between the particle and the tip. In addition, the results indicate that when the AFM tip is irradiated by the annular laser with higher frequency, the near-field enhancement underneath the apex of the tip increases correspondingly. Moreover, when the irradiation angle is 60 Deg and the irradiation distance is 65 nm, the near-field enhancement reaches the peak. Based on the numerical simulation of the near-field enhancement, the system of the AFM tip irradiated by annular laser beam can be anticipated for further application in the field of nanofabrication.

Ultranarrow Second-Harmonic Resonances in Hybrid Plasmon-Fiber Cavities.

We demonstrate second-harmonic generation with ultranarrow resonances in hybrid plasmon-fiber cavities, formed by depositing single-crystalline gold nanorods onto the surface of tapered microfibers with diameters in the range of 1.7-1.8 μm. The localized surface plasmon mode of the single gold nanorod efficiently couples with a whispering gallery mode of the fiber, resulting in a very narrow hybrid plasmon-fiber resonance with a high quality factor Q of up to 250. When illuminated with a tunable 100 fs laser, a sharp SHG peak narrower than half of the spectral width of the impinging laser emerges, superimposed on a broad multiphoton photoluminescence background. The enhancement of the SHG peak of the hybrid system is typically 1000-fold when compared to that of a single gold nanorod alone. Tuning the laser over the hybrid resonance enables second-harmonic spectroscopy and yields an ultranarrow line width as small as 6.4 nm. We determine the second-harmonic signal to rise with the square of the laser power, while the multiphoton photoluminescence background rises with powers between 4 and 6, indicating a very efficient higher-order process. A coupled anharmonic oscillator model is able to describe the linear as well as second-harmonic resonances very well. Our work will open the door to the simultaneous utilization of narrow whispering gallery resonances together with high plasmonic near-field enhancement and should allow for nonlinear sensing and extremely efficient nonlinear light generation from ultrasmall volumes.

Mechanically reconfigurable architectured graphene for tunable plasmonic resonances

Graphene nanostructures with complex geometries have been widely explored for plasmonic applications, as their plasmonic resonances exhibit high spatial confinement and gate tunability. However, edge effects in graphene and the narrow range over which plasmonic resonances can be tuned have limited the use of graphene in optical and optoelectronic applications. Here we present a novel approach to achieve mechanically reconfigurable and strongly resonant plasmonic structures based on crumpled graphene. Our calculations show that mechanical reconfiguration of crumpled graphene structures enables broad spectral tunability for plasmonic resonances from mid- to near-infrared, acting as a new tuning knob combined with conventional electrostatic gating. Furthermore, a continuous sheet of crumpled graphene shows strong confinement of plasmons, with a high near-field intensity enhancement of ~1 × 104. Finally, decay rates for a dipole emitter are significantly enhanced in the proximity of finite-area biaxially crumpled graphene flakes. Our findings indicate that crumpled graphene provides a platform to engineer graphene-based plasmonics through broadband manipulation of strong plasmonic resonances.

Numerical Analysis of Nanogap Effects in Metallic Nano-disk and Nano-sphere Dimers: High Near-field Enhancement with Large Gap Sizes

Various gap sizes are investigated numerically to extract the local electric field enhancement from a gold sphere and disk dimer systems. Our simulations predict that a metallic disk dimer system(s) exhibit large local electric field enhancements at larger gap sizes (20 nm, 40 nm) as compared to that of sphere dimer designs (gap size = 8 nm, 14 nm). These gap size differences ~ 2.5 -3.3 times larger as compared to that of sphere dimer systems, facilitates the device fabrication. These numbers are obtained by the influence of uniform gap size distribution as a function of total volume “±Z direction”. Such geometry, by achieving good local electric field enhancement from larger gap size, will enhance the variety of potential applications in the field of plasmonics, sensors, single molecule detection, surface enhanced spectroscopy, and so on.

Effects of Dielectric Spacer on Absorbance Characteristics of a Dual-Band Nanoaperture Based Perfect Absorber

In this study, a novel perfect absorber (PA) array based on H-shaped nanoapertures for bio-sensing applications in infrared regime is presented. Proposed PA array has a dual-band spectral response, and the locations of these resonances can be adjusted by varying the geometrical dimensions and layer thicknesses of the structure. Nearly unity absorbance is obtained from the PA array for both resonances. The structure design is based on the near field plasmon coupling between the gold film layer and the top nanoaperture array. In this context, the dielectric spacer layer is used to support this plasmon coupling and the gold film on the silicon substrate is also utilized to eliminate the transmittance through the structure. Different dielectric spacers (MgF2, SiO2, and Al2O3) are used to investigate the effects of dielectric spacer on the absorbance characteristics of proposed PA array. High field enhancement is achieved by the interaction of the sharp corners of nanoapertures. The near field enhancements are more than 1500 times at the first resonance frequency, more than 1000 times at the second resonance frequency which is highly desirable for the infrared bio-sensing applications. Due to the high near-field enhancement and nearly unity absorbance, the proposed dual-band PA array with adjustable spectral responses can be useful for bio-sensing applications in infrared regime.

Four-Headed Arrow Shaped Dual Band Perfect Absorbers for Biosensing Applications

In this study, a novel perfect absorber (PA) array based on four-headed arrow nanoparticles for biosensing applications in mid-infrared regime is presented. Proposed PA array has a dual-band spectral response, and the locations of resonances can be adjusted by varying the geometrical dimensions of the structure. Nearly unity absorbance is obtained from the PA array for both resonances. Different dielectric spacers (MgF 2 , SiO 2 , and Al 2 O 3 ) are used to investigate the effects of dielectric spacer on the absorbance characteristics of proposed PA array. Absorbance characteristics of PA array are analyzed by using finite difference time domain (FDTD) method. High field enhancement is achieved by the interaction of the sharp corners of arrow nanoparticles. Linear correlation between the resonance frequencies and the refractive index of cladding mediums is determined. Due to the high refractive index sensitivity and near-field enhancement, and nearly unity absorbance, the proposed dual-band PA array with adjustable spectral responses can be useful for biosensing applications in mid-infrared regime.

Polarization insensitive plasmonic perfect absorber with coupled antisymmetric nanorod array

Plasmonic perfect absorbers have a large potential for biomolecular sensing applications such as refractometric biosensing and surface enhanced spectroscopy due to their high near-field enhancement and near-unity absorption capabilities. Despite these properties, novel plasmonic perfect absorbers with additional features are highly needed for real-world sensing applications. In this context, we present the design, characterization and experimental realization of a subwavelength plasmonic perfect absorber array with crucial properties including the dual-band absorption, high near-field enhancement with a large number of hot-spots, mass-production compatible design, high sensitivity, and polarization independency for both narrow and wide resonance bands. In order to investigate the spectral characteristics and to obtain a fine-tuning mechanism, we perform several numerical analyses by using finite difference time domain method. We also investigate the charge and current density distributions at the corresponding resonances. It is observed that the physical origin of the dual-band behavior is based on the dipolar plasmonic resonance of the horizontal nanorods and the plasmonic coupling mechanism of the structure. Furthermore, we investigate the sensing performance of the proposed device by calculating the refractive index sensitivity and figure-of-merit for the resonant modes by embedding it into different cladding media with various refractive indices. Finally, in order to demonstrate the potential of the proposed plasmonic perfect absorber for surface enhanced infrared absorption spectroscopy, we simulate the detection of carbonyl v(CO) stretching mode of a conformal poly(methyl methacrylate) layer. Due to its highly desired properties, the proposed device can be a good candidate for biomolecular sensing applications and also the results of our investigation can be useful for the further designs of plasmonic based sensors.

Plasmonic colloidal nanoparticles with open eccentric cavities via acid-induced chemical transformation

Surface-enhanced Raman spectroscopy (SERS) has been considered a promising technique for the detection of trace molecules in biomedicine and environmental monitoring. The ideal metal nanoparticles for SERS must not only fulfill important requirements such as high near-field enhancement and a tunable far-field response but also overcome the diffusion limitation at extremely lower concentrations of a target material. Here, we introduce a novel method to produce gold nanoparticles with open eccentric cavities by selectively adapting the structure of non-plasmonic nanoparticles via acid-mediated surface replacement. Copper oxide nanoparticles with open eccentric cavities are first prepared using a microwave-irradiation-assisted surfactant-free hydrothermal reaction and are then transformed into gold nanoparticles by an acidic gold precursor while maintaining their original structure. Because of the strong near-field enhancement occurring at the mouth of the open cavities and the very rough surfaces resulting from the uniformly covered hyperbranched sharp multi-tips and the free access of SERS molecules inside of the nanoparticles without diffusion limitation, adenine, one of the four bases in DNA, in an extremely diluted aqueous solution (1.0 pM) was successfully detected with excellent reproducibility upon laser excitation with a 785-nm wavelength. The gold nanoparticles with open eccentric cavities provide a powerful platform for the detection of ultra-trace analytes in an aqueous solution within near-infrared wavelengths, which is essential for highly sensitive, reliable and direct in vivo analysis. Scientists have made colloidal gold nanoparticles with open cavities that have an intense surface-enhanced Raman spectroscopy (SERS) signal. The researchers, who are in Korea, prepared the nanoparticles by producing copper oxide nanoparticles having very rough surfaces and an open cavity through a microwave-assisted hydrothermal reaction. They then converted them into gold nanoparticles with the same structure by using an acidic gold precursor. As the rough surface and open cavity mouth greatly enhance the near field, the nanoparticles produce a very strong SERS signal. The researchers demonstrated the potential of the nanoparticles by using them to detect adenine, one of the four bases of DNA, in an extremely low concentration (1.0 picomolar) solution. They anticipate that the nanoparticles will be valuable for detecting very low concentrations of analytes in aqueous solutions using near-infrared excitation. Plasmonic gold nanoparticles with open eccentric cavity are fabricated by selectively adapting the structure of non-plasmonic nanoparticles via acid-induced chemical transformation. Because of strong near-field enhancement occurring at the mouth of the open cavity and very rough surfaces resulting from uniformly covered hyperbranched sharp multi-tips as well as free access of SERS molecules inside the nanoparticles without diffusion limitation, adenine, one of four bases in DNA, in an extremely diluted aqueous solution (1.0 pM) was successfully detected with excellent reproducibility at a laser excitation with 785 nm wavelength.

Nanoparticle-Based Metamaterials as Multiband Plasmonic Resonator Antennas

Plasmonic metamaterials based on metal-dielectric nanostructures exhibit unique optical properties such as high near-field enhancement, negative refractive indexing, and optical cloaking. In this paper, we present a plasmonic multiband metamaterial based on UT shaped nanoparticles. In order to understand the multispectral response, we analyze the near-field distributions at the corresponding resonance frequencies. In addition, we both numerically and experimentally, show the dependence of the spectral response on the geometrical parameters of the structure. By embedding the system in a dielectric cladding medium, we show strong sensitivities of the resonant behavior to the refractive index and thickness of the dielectric load. Due to its tunable multiband spectral characteristics, the proposed metamaterial antenna can be used for wide range of applications, such as wavelength-tunable active filters, optical modulators, ultrafast switching devices, and biosensing.

Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source

We demonstrate the possibility to achieve optical triggering of photochemical reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.

Terahertz transparency at Fabry-Perot resonances of periodic slit arrays in a metal plate: experiment and theory

We report on a perfect transmission in one-dimensional metallic structure using time-domain terahertz spectroscopy. Fabry-Perot resonance appearing in spectral region below first Rayleigh minimum strongly enhances transmission up to over ninety-nine percent. Theoretical calculations reveal that under the perfect transmission condition, a symmetric eigenmode inside the slits is excited and a funneling of all incident energy onto the slits occurs, resulting in large energy concentration equivalent to the inverse sample coverage and high near-field enhancement of electric and magnetic field intensities. Our work opens way toward near-field terahertz amplification, applicable to high-field terahertz spectroscopy.